Chemical structure and biological activity 



(ii) No convincing arguments have been adduced in support of the view 

 that certain separate positions in the nucleus have a special binding function. 

 For the moment the most simple and plausible explanation of the observed 

 facts is that the nucleus with its substituents (other than COOH) as a whole 

 acts as an attaching unit. 



(iii) Substituents in 3- and/or 5-positions are allowed (or even desired as 

 they mostly enhance activity as compared with 2:6-di-substitutionj. The 

 restrictions as to their size are less than those for the o-substituents. 



(iv) All 4-substituents larger than F are incompatible with activity. 



(a) 



ic) 



ib) 



id) 



Figure 4. Pea test. 



(a) XI. CHg: 

 {b) XI. CI: 

 (c) XL Br: 

 {d) XI. I: 



25, 10. 4, lAO-^ mollL 

 25. 10, 4, 1.10-5 mo///. 

 25, 10, 4, 1 .10-" mol II. 

 25, 10,4, 1.10-5 mo///. 



(v) All requirements with respect to the substituents strongly suggest that 

 they imply the absence of hindrances to the fitting of the molecule on to its 

 receptor, rather than the presence of specific binding spots, j 



The weak activity of a-naphthoic acid (IX) and its enhancement by 

 2-substituents (e.g. X) has been explained by us on the same basis as the 

 activity of the diortho substituted benzoic acids (Veldstra, 1952). Studies of 



t In the discussions about the kinetics of auxin action and the two-point attachment 

 theory of Bonner (1954), illustrations are used in which besides the COOH group a special 

 position in the active molecule is bound to the receptor. These illustrations are misleading as, 

 apart from the fact that the calculations are based in fact on the assumption that the bindings 

 are spontaneously reversible (so normal chemical bonds must be excluded!), the possible 

 validity of these calculations has nothing to do with two points in the molecule. It would 

 merely imply that two factors are operating, without informing us in the least about their nature. 



122 



